X-ray generating apparatus (X-ray tubes) are used in the industrial field, medical field and so on, and are used in nondestructive inspection system, for example. The X-ray tubes mounted in the nondestructive inspection system are divided broadly into a open type X-ray tube and a sealed type X-ray tube. The open type X-ray tube has a structure for vacuuming a housing using a turbo-molecular pump, for example, and enables changing of consumables such as a filament forming a cathode and a target. The sealed type X-ray tube does not require a vacuum pump, but has a vacuum-locked housing. Of the above types, the sealed X-ray tube includes an electron gun often having, mounted therein, a flat cathode similar to that used in a cathode-ray tube from the viewpoint of long-term stability.
FIG. 6 is a schematic view showing an electron beam extracting portion of a flat cathode. As shown in FIG. 6, two or more intermediate electrodes are arranged between a cathode 102a which emits an electron beam B, and a target. These intermediate electrodes are referred to as a first electrode 102b and a second electrode 102c in order from adjacent the cathode 102a. With the potential of the cathode 102a serving as reference potential, a negative potential is applied to the potential of the first electrode 102b and a positive potential to the second electrode 102c. The electron beam B emitted from the cathode 102a forms a crossover (virtual source) adjacent these electrodes (see sign “DS” in FIG. 6).
A schematic view of a conventional X-ray tube with this electron gun mounted therein is shown in FIG. 7 or 8. As shown in FIG. 7 or 8, an X-ray tube T includes a vacuum housing 101 containing an electron gun 102 and a target 103, causes an electron beam B emitted from the electron gun 102 to collide with the target 103, and takes X rays generated from a position of collision out through an X-ray window 101b provided on the vacuum housing 101. The electron gun 102 has a cathode 102a which emits the electron beam B, and intermediate electrodes such as a first electrode 102b and a second electrode 102c. Since it is necessary to project the above crossover image with a desired focus diameter onto the target 103, an actual X-ray tube further includes a third electrode (also called a “focusing electrode”) 102d as an intermediate electrode to form electron optics.
Because of the structure for assembling the X-ray tube T, the cathode 102a and first electrode 102b, and the first electrode 102b and second electrode 102c, of the electron gun 102 are mechanically interconnected, respectively. Since it is necessary to apply different potentials independently, the cathode and each electrode are assembled, for example, through an electric insulator such as alumina, sapphire or bead glass. As a method of applying potentials to the cathode and each electrode, as shown in FIG. 7 or 8, the potentials are applied from outside the X-ray tube T by electrically and mechanically connecting pins 105 of a stem 104 (see FIG. 7 or 8) and the objective electrodes through thin struts or ribbon electrodes 106 (see FIG. 8). Since a potential at a maximum of several kilovolts may be applied to these electrodes and cathode, the portion of electron gun 102 and the vacuum housing 101 are isolated by at least about 1 mm space gap. The vacuum housing 101 is grounded. As what specifies the potentials of these electrodes, a method has been proposed in which the above third electrode is placed in contact with the vacuum housing to have the same potential (see Patent Documents 1 and 2, for example).